Field of the Invention
The present disclosure relates to a heater, an image heating device, and an image forming apparatus.
Description of the Related Art
As an image heating device, there is a device that has a tubular film, a heater which is in contact with an inner surface of the film, and a roller which forms a nip portion with the heater via the film. In an image forming apparatus of an electrophotographic system in which the image heating device is mounted as a fixing device, when fixing processing is continuously performed on recording materials each of which has a size smaller than a maximum sheet passing width in a direction orthogonal to a conveyance direction of the recording material, so-called temperature rise of a non-sheet-passing portion is caused. That is, a phenomenon that temperature of each part in a region (non-sheet-passing portion) through which no recording material passes in a longitudinal direction of a fixing nip portion gradually rises is caused. An image heating device is required to prevent temperature of the non-sheet-passing portion from exceeding a withstanding temperature limit of each member in the device. Therefore, a method of suppressing the temperature rise of the non-sheet-passing portion by reducing throughput (the number of sheets for which printing can be performed per one minute) (throughput down) of continuous printing is often used.
As a method of suppressing the temperature rise of the non-sheet-passing portion, a device in which a heat generating resistor on a heater is divided into a plurality of groups (heating blocks) in a longitudinal direction of the heater and heat generation distribution of the heater is switched in accordance with a size of a recording material is proposed (Japanese Patent Laid-Open No. 2014-59508).
However, temperature distribution of one heating block is affected by a heat generation state of another heating block. Particularly, temperature of an edge region of the one heating block in the longitudinal direction of the heater is affected by a heat generation state or a temperature state of an adjacent heating block. As a result, the temperature distribution in the one heating block becomes uneven, causing a possibility of fixing failure of a toner image.